Big cheeses and high achievers line the walls of the National Portrait Gallery, confident that its fine, high rooms in St Martin's Place are their rightful home. Their gold-rimmed images are the nation's portraits, it is true, but they do not add up to a portrait of the nation. Where are the people?

Where are those that bore its children, did its work and fought its wars?

Their absence is not altogether the gallery's fault. Few pictures of individuals from history's masses exist.

Some, of course, are found in the backgrounds of battles, and here and there a peasant from central casting has a trudge-on part in a landscape.

Even early photography was the province of the elite. The real democratisation of portraiture had to wait until 1902, when the Post Office allowed the picture postcard to have a message on one side and a picture on the other.

Suddenly in every town and village, photographic studios sprang up offering backgrounds of baronial splendour or pastoral idyll, against which, one after another, citizens came to pose in ornate chairs or stand among potted plants.

Pets were brought in and the owner of a new bicycle would proudly display it.

A voluntary visual census took place as everyone became a postcard.

My mother gave me my father's postcard portrait - a record of his welterweight boxing heyday, fists clenched, circa 1906. A generation or two later, millions of such images started spilling from discarded albums into the world of junk shops and flea markets, losing in the process context and identity. Soon they reached the specialised postcard fairs which take place every week up and down the country in characterless municipal leisure centres and dingy hotels.

This is where I came in, as the addicted collector, eager for a new and inexhaustible source of pictorial riches.

Most who attend such gatherings are in search of postcards of particular places or the productions of sugary Edwardian illustrators; my quest is conducted at the bottom end of the market, in the 25p boxes of the dealers.

There I find most of these defining images of real presences portrayed in vernacular stylelessness.

Characterful people gaze out as if from an eternal present, their indiv iduality transcending the anonymity they have acquired. On occasion, there is a name scrawled across a card, and perhaps a date ("Mrs Clark, aged 109" is the oldest person, earliest born, in my collection, on a card produced in 1904).

I have combed through more than two million postcards and have ended up buying (at prices ranging from 10p to £10) 50,000 of them. It is at this point that the chronic stage in the psychopathology of collecting is reached, the delightful drudgery of sorting.

As yet there is no official taxonomy of these cards and I am free to invent my own headings under which to group them. Since the heyday of the postcard was the first third of the century, the words of music-hall songs provide clues for categories. "I do like to be beside the seaside" was an obvious starting point, though this huge subject quickly got subdivided into Promenades, Bathers, the Pier, the Beach and others.

Other titles evoke their corresponding pictures: "I dreamed I dwelt in marble halls" captures the mood of lavish studio backdrops, whereas My Old Dutch ("We've been together now for 40 year") personifies Darby and Joan couples who recorded their existence in the nick of time. Some cards echo darker themes, such as those of First World War soldiers in their new uniforms pictured with their families, perhaps for the last time ("Wish me luck as you wave me goodbye").

Although many groupings of cards were predictable (Weddings, Sport), others emerged by sheer force of numbers.

The ubiquitous aspidistra, the parlour palm, was a requisite of interiors and commanded a section of its own, though few seem to match the song made famous by Gracie Fields, "The biggest aspidistra in the world".

All levels of make-believe were served by postcard studios. Today's obsession with virtual reality had its equivalent in the area of fantasy transport, where those who would never fly in an aeroplane and probably had not even travelled in a car could sail away into the studio skies in sketchy cardboard replicas of aircraft, or set off from their background mansion in plywood limousines.

There was a brief vogue for cinema studios where you could be photographed in the style of silentscreen stars, with glamorous lighting and appropriate props. The longestrunning caterer for such dreams was the Fancy Dress Studios at 31 Oxford Street, which offered, until the late Thirties, stage sets for anything from Wild West dramas to nightclub encounters. Its wardrobe also offered slightly risque pyjama scenes and (oddly popular among women) military cross-dressing.

Postcard photography was not limited to the studio. Enterprising local professionals toured the streets of every town, making portraits of houses with their occupants gathered outside ("Ours is a nice house, ours is") or, for the more discreet, of garden groups. At the seaside, there was a rapid trade in ready-bynextday postcards and photographers visited factories and offices. Now that a wedding is more likely to be the subject of a video, the school photographer is the last link with that itinerant world.

Of the categories featured in the NPG show, my favourite is that of the women who took on the jobs of those bus conductors, postmen and others who went to the trenches in the First World War. They wear the uniforms with pride, as do the girls of the Land Army, and that elite of women volunteers, the Munition Workers, in their bold trousered outfits, well aware that an irreversible moment had arrived in the emancipation of women. These more than any embody for me the spirit of a nation and the essence of progress. A constant theme of the cards in the exhibition is modernity: though they come from the past they urgently depict their present and carry it forward into our future.